Detecting changes in an environmental condition along a wellbore

Abstract

A method and system can include positioning an optical waveguide along a wellbore, and launching one or more optical signals into the waveguide at one or more optical signal frequencies and during one or more time periods, thereby resulting in one or more backscattered signals being received by the receiver, which produces a trace for each of the one of more backscattered signals. Changing an environmental condition in the wellbore, generating additional backscattered light signals at one or more frequencies after the change. Comparing the traces generated before the condition change to those generated after the change, identifying a before trace and an after trace that are substantially equal to each other and identifying a frequency difference between these traces. The frequency difference can be used to determine the amount of change in the environmental condition that occurred when the environmental change event happened.

The invention claimed is: 
     
       1. A method of detecting changes in an environmental condition along a wellbore, the method comprising:
 positioning an optical waveguide along the wellbore; 
 introducing an optical signal from a light source into the optical waveguide at a first frequency during a first time period; 
 receiving a backscattered signal from the optical waveguide in response to the introduction of the optical signal during the first time period; 
 producing a first trace which represents an intensity of the backscattered signal along the waveguide; 
 causing a change in the environmental condition; 
 adjusting the first frequency of the light source to a next frequency which is different from the first frequency; 
 introducing an optical signal from the light source into the optical waveguide at the next frequency during a next time period; 
 receiving a next backscattered signal from the optical waveguide in response to the introduction of the optical signal during the next time period; 
 producing a next trace which represents an intensity of the next backscattered signal along the waveguide during the next time period; 
 identifying differences between frequencies associated with the first and next traces; and 
 determining the change in the environmental condition based on the differences when the first and next traces are substantially equal to each other. 
 
     
     
       2. The method of  claim 1 , wherein the determining further comprises, calculating a difference in frequency between the first and next frequencies, and calculating the change in the environmental condition based on the calculated difference in frequency. 
     
     
       3. The method of  claim 1 , wherein the adjusting, the introducing the optical signal at the next frequency, the receiving the next backscattered signal in response to the introduction of the optical signal at the next frequency, and the identifying the differences are repeated until the first trace substantially equals the next trace. 
     
     
       4. The method of  claim 1 , wherein the intensity represents the environmental condition along the wellbore. 
     
     
       5. The method of  claim 4 , wherein the environmental condition is temperature. 
     
     
       6. The method of  claim 1 , wherein the first frequency comprises multiple frequencies and the first trace comprises multiple traces with each one of the first traces corresponding to a separate one of the first frequencies, and with each of the first frequencies being different from other ones of the first frequencies. 
     
     
       7. The method of  claim 6 , wherein the identifying further comprises identifying differences between frequencies associated with each one of the first traces and the next trace, and wherein the determining further comprises determining the change in the environmental condition based on the differences when at least one of the first traces is substantially equal to the next trace. 
     
     
       8. The method of  claim 1 , wherein causing the change in the environmental condition further comprises:
 opening a valve to increase fluid flow into the wellbore from a production zone; or 
 closing a valve to decrease fluid flow into the wellbore from the production zone. 
 
     
     
       9. The method of  claim 1 , wherein:
 the determining further comprises calculating at least one of a differential fluid pressure, a fluid flow rate, and a fluid composition based on the determined change in the environmental condition; or 
 along the wellbore which comprises multiple segments, and wherein the first and next traces represent the environmental condition along a length of at least one of the multiple segments. 
 
     
     
       10. A system that detects a change in an environmental condition along a wellbore, the system comprising:
 an optical waveguide positioned in the wellbore; 
 a light source that introduces an optical signal into the waveguide; and 
 a receiver that receives a backscattered signal from the optical waveguide and produces a trace which represents an intensity of the backscattered signal along the optical waveguide, wherein the intensity represents the environmental condition along the wellbore, 
 wherein the light source introduces light at a first frequency into the optical waveguide during a first time period and the receiver produces a first trace in response to reception of the backscattered signal from the waveguide, 
 wherein the light source introduces light at a second frequency into the optical waveguide during a second time period and the receiver produces a second trace in response to reception of the backscattered signal from the optical waveguide, 
 wherein the second frequency is different from the first frequency, 
 wherein the first trace is substantially equal to the second trace, and 
 a change in the environmental condition along the wellbore is calculated based on the difference between the first and second frequencies. 
 
     
     
       11. The system according to  claim 10 , wherein the change in the environmental conditions is due to a valve that is selectively opened and closed, which variably restricts fluid flow through the wellbore. 
     
     
       12. The system according to  claim 11 , wherein:
 the valve is closed for the first time period and the valve is opened for the second time period; or 
 the valve is opened for the first time period and the valve is closed for the second time period. 
 
     
     
       13. The system according to  claim 10 , wherein at least one of a differential fluid pressure, a fluid flow rate, and a fluid composition of a fluid flowing from a production zone into the wellbore is determined based on the difference between the first and second frequencies. 
     
     
       14. The system according to  claim 10 , wherein:
 the traces are coherent optical time domain reflectometry traces; or 
 along the wellbore which comprises multiple segments, and wherein the first and second traces represent environmental conditions along a length of at least one of the multiple segments. 
 
     
     
       15. The system according to  claim 10 , wherein the first frequency comprises multiple frequencies, the first trace comprises multiple traces, and the first time period comprises multiple time periods, with each of the first traces associated with one of the first time periods and one of the first frequencies, wherein one of the first traces is substantially equal to the second trace, and the change in the environmental condition along the wellbore is calculated based on a difference between the frequency associated with the one of the first traces and the second frequency. 
     
     
       16. The system according to  claim 15 , wherein the second frequency comprises multiple frequencies, the second trace comprises multiple traces, and the second time period comprises multiple time periods, with each of the second traces associated with one of the second time periods and one of the second frequencies, wherein one of the second traces is substantially equal to the one of the first traces, and the change in the environmental condition along the wellbore is calculated based on a difference between the frequency associated with the one of the first traces and the frequency associated with the one of the second traces. 
     
     
       17. The system according to  claim 10 , wherein the second frequency comprises multiple frequencies, the second trace comprises multiple traces, and the second time period comprises multiple time periods, with each of the second traces associated with one of the second time periods and one of the second frequencies, wherein one of the second traces is substantially equal to the first trace, and the change in the environmental condition along the wellbore is calculated based on a difference between the frequency associated with the one of the second traces and the first frequency. 
     
     
       18. A method for detecting a change in an environmental condition along a wellbore, the method comprising:
 positioning an optical waveguide along the wellbore; 
 introducing each one of first optical signals from a light source into the optical waveguide at one of multiple first frequencies during a first time period; 
 receiving first backscattered signals from the optical waveguide in response to the introduction of the first optical signals and producing a set of first traces; 
 making a set of baseline traces equal to the set of the first traces; 
 initiating a change in the environmental condition; 
 introducing each one of second optical signals from the light source into the optical waveguide at one of multiple second frequencies after at least a portion of the environmental condition has occurred; 
 receiving second backscattered signals from the optical waveguide in response to the introduction of the second optical signals and producing a set of second traces; 
 comparing the baseline traces to the second traces; 
 determining that at least one of the baseline traces correlates to at least one of the second traces; 
 determining an incremental change in the environmental condition based on differences between frequencies that are associated with the at least one of the baseline traces and the at least one of the second traces; 
 adjusting the multiple second frequencies; 
 repeating 1) the introducing the second optical signals, 2) the receiving the second backscattered signals, 3) the comparing the baseline traces to the second traces, 4) the determining the correlation, 5) the determining the incremental change and 6) the adjusting the multiple second frequencies until the environmental condition is stable; and 
 combining the incremental environmental condition changes to determine a total environmental change in the wellbore. 
 
     
     
       19. The method of  claim 18 , wherein the determining the correlation further comprises determining that none of the baseline traces correlate with any of the second traces, making the set of baseline traces equal to a last set of the second traces that included at least one of the second traces that did correlate to at least one of the baseline traces. 
     
     
       20. The method of  claim 18 , further comprising determining at least one of the group consisting of fluid type, fluid composition, fluid flow, and fluid pressure differential based on the total environmental condition change in the wellbore.